Structures and methods for a field-reset spin-torque MRAM

ABSTRACT

An apparatus and method of programming a spin-torque magnetoresistive memory array includes a metal reset line positioned near each of a plurality of magnetoresistive bits and configured to set the plurality of magnetoresistive memory elements to a known state by generating a magnetic field when an electrical current flows through it. A spin torque transfer current is then applied to selected ones of the magnetoresistive bits to switch the selected bit to a programmed state. In another mode of operation, a resistance of the plurality of bits is sensed prior to generating the magnetic field. The resistance is again sensed after the magnetic field is generated and the data represented by the initial state of each bit is determined from the resistance change. A spin torque transfer current is then applied only to those magnetoresistive bits having a resistance different from prior to the magnetic field being applied.

TECHNICAL FIELD

The exemplary embodiments described herein generally relate to magnetoresistive memories and more particularly to structures and methods of reading and programming a spin-torque magnetoresistive random access memory (MRAM) having a field-reset capability.

BACKGROUND

Magnetoelectronic devices, spin electronic devices, and spintronic devices are synonymous terms for devices that make use of effects predominantly caused by electron spin. Magnetoelectronics are used in numerous information devices to provide non-volatile, reliable, radiation resistant, and high-density data storage and retrieval. The numerous magnetoelectronics information devices include, but are not limited to, Magnetoresistive Random Access Memory (MRAM), magnetic sensors, and read/write heads for disk drives.

Typically an MRAM includes an array of magnetoressistive memory elements. Each magnetoresistive memory element typically has a structure that includes multiple magnetic layers separated by various non-magnetic layers, such as a magnetic tunnel junction (MTJ), and exhibits an electrical resistance that depends on the magnetic state of the device. Information is stored as directions of magnetization vectors in the magnetic layers. Magnetization vectors in one magnetic layer are magnetically fixed or pinned, while the magnetization direction of another magnetic layer may be free to switch between the same and opposite directions that are called “parallel” and “antiparallel” states, respectively. Corresponding to the parallel and antiparallel magnetic states, the magnetic memory element has low and high electrical resistance states, respectively. Accordingly, a detection of the resistance allows a magnetoresistive memory element, such as an MTJ device, to provide information stored in the magnetic memory element. There are two completely different methods used to program the free layer: field switching and spin-torque switching. In field-switched MRAM, current carrying lines adjacent to the MTJ bit are used to generate magnetic fields that act on the free layer. In spin-torque MRAM, switching is accomplished with a current pulse through the MTJ itself. The angular momentum carried by the spin-polarized tunneling current causes reversal of the free layer, with the final state (parallel or antiparallel) determined by the polarity of the current pulse. Spin-torque transfer is known to occur in MTJ devices and giant magnetoresistance devices that are patterned or otherwise arranged so that the current flows substantially perpendicular to the interfaces, and in simple wire-like structures when the current flows substantially perpendicular to a domain wall. Any such structure that exhibits magnetoresistance has the potential to be a spin-torque magnetoresistive memory element.

Spin-torque MRAM (ST-MRAM), also known as spin-torque-transfer RAM (STT-RAM), is an emerging memory technology with the potential for non-volatility with unlimited endurance and fast write speeds at much higher density than field-switched MRAM. Since ST-MRAM switching current requirements reduce with decreasing MTJ dimensions, ST-MRAM has the potential to scale nicely at even the most advanced technology nodes. However, increasing variability in MTJ resistance and sustaining relatively high switching currents through bitcell select devices in both current directions can limit the scalability of ST-MRAM. The write current is typically higher in one direction compared to the other, so the select device must be capable of passing the larger of the two currents. In addition, ST-MRAM switching current requirements increase as the write current pulse duration is reduced. Because of this, the smallest ST-MRAM bitcell approach may require relatively long switching times.

One approach to manage the increasing MTJ resistance variability as ST-MRAM is scaled is to use a self-referenced read operation to determine the state of the bits. One such self-referenced read operation would bias a bit to a desired voltage and maintain a reference reflecting the required current, then switch the bit to a known state. The original state of the bit can be determined by comparing the current required to bias the bit in the new state to the current that was required in the original state. No change in current would indicate that the original state matched the switched state, while a change in current in the expected direction would indicate that the original state was opposite of the switched state.

Where a self-referenced read operation can overcome the impact of MTJ resistance variation, it may also increase the read time requirement. The relatively long time required for a self-referenced read operation in conjunction with the relatively long ST-MRAM write times (compared to static random access memory (SRAM), for example) make high sequential bandwidth read and write approaches, like those used by dynamic random access memories (DRAMs), more desirable. In the approach used by DRAMs, a large page of bits (several thousand) are read simultaneously and the values are each stored in a latch, then at much higher speed, subsets of the page of bits are read out of the part. For writes in DRAMs, data is similarly written to subsets of the page, after which there is a time required to complete the write process. DRAM interfaces, such as synchronous dynamic random access memory (SDRAM) and double data rate (DDR) DRAM, are designed to maximize sequential bandwidth while allowing for the naturally slow random cycle time of DRAM technology. ST-MRAM may benefit from a similar approach; however, one key challenge would be managing the power required to read and write a large page of bits at high bandwidth.

Accordingly, it is desirable to provide ST-MRAM structures and methods that shorten a self-referenced read operation, reduce the power requirement when performing a self-referenced read operation to a plurality of bits, or enable a memory to function with ST-MRAM switching that only requires current be applied through the MTJ in the one direction having the lower critical current requirement. Furthermore, other desirable features and characteristics of the exemplary embodiments will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.

BRIEF SUMMARY

A method and apparatus are provided for programming and reading a spin-torque magnetoresistive random access memory.

The apparatus comprises a plurality of magnetoresistive memory elements which are typically MTJ bits with two stable magnetic states, a metal reset line positioned adjacent each of the plurality of magnetoresistive memory elements and configured to set the plurality of magnetoresistive memory elements to a known state by generating a magnetic field when an electrical current of predetermined magnitude and direction flows through the metal reset line, a bit line coupled to the plurality of magnetoresistive memory elements, and circuitry coupled to the bit line and configured to apply a spin torque transfer current through the bit line to selected ones of the plurality of magnetoresistive memory elements. Additional associated circuitry stores the data that is that is read from and written to the array of magnetoresistive memory elements, and enables access to that stored data through an interface.

An exemplary method embodiment includes setting the plurality of magnetoresistive memory elements to a first state by applying a current through the metal reset line to apply a magnetic field to each of the plurality of magnetoresistive memory elements, and programming selected ones of the plurality of magnetoresistive memory elements to a second state by applying a spin torque transfer current through the selected magnetoresistive memory elements from the circuitry to the bit line.

Another exemplary method embodiment includes sensing a resistance for each of the plurality of magnetoresistive memory elements, setting the plurality of magnetoresistive memory elements to a first state by applying a current through the metal reset line to apply a magnetic field to each of the plurality of magnetoresistive memory elements, sensing of a resistance for each of the magnetoresistive memory elements, and programming the magnetoresistive memory elements determined to have changed back to the second state by applying a spin torque transfer current through the selected magnetoresistive memory elements from the circuitry to the bit line.

In yet another exemplary embodiment, sensing a resistance for each of the plurality of magnetoresistive memory elements, setting the plurality of magnetoresistive memory elements to a first state by applying a current through the metal reset line to apply a magnetic field to each of the plurality of magnetoresistive memory elements, sensing of a resistance of each of the magnetoresistive memory elements, identifying and storing data representing the initial state of each of the magnetoresistive memory elements as determined from the change in resistance subsequent to the setting step, changing one or more of the stored data; and programming the selected ones of the plurality of magnetoresistive memory elements to a second state in response to the stored data.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and

FIG. 1 is a typical field-switched MRAM memory element array;

FIG. 2 illustrates magnetic fields generated at a memory element of a typical field-switched MRAM memory element array;

FIG. 3 is a typical spin-torque memory cell;

FIG. 4 is an exemplary embodiment of two cells of an MRAM integrated array;

FIG. 5 is another exemplary embodiment of two cells of a MRAM integrated array;

FIG. 6 is yet another exemplary embodiment of two cells of a MRAM integrated array;

FIG. 7 is a flow chart of a method of operation for programming the MRAM integrated arrays of FIGS. 4, 5, and 6;

FIG. 8 is flow chart of a method of operation for reading the MRAM integrated arrays of FIGS. 4, 5, and 6;

FIG. 9 is a flow chart of a method of operation for reading and programming the MRAM integrated arrays of FIGS. 4, 5, and 6;

FIG. 10 is a block diagram of an exemplary embodiment including a sense amplifier and write circuit.

FIG. 11 is a schematic diagram of an exemplary embodiment including NMOS transistors for providing a unidirectional read current and a spin torque transfer current within the MRAM integrated arrays of FIGS. 4 and 6;

FIG. 12 is a schematic diagram of an exemplary embodiment including diodes for providing a unidirectional read current and a spin torque transfer current within the MRAM integrated arrays of FIGS. 4 and 6; and

FIG. 13 is a schematic diagram of an exemplary embodiment including bipolar transistors for providing a unidirectional read current and a spin torque transfer current within the MRAM integrated arrays of FIGS. 4 and 6.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the subject matter or the application and uses of such embodiments. Any implementation described herein as exemplary is not necessarily to be construed as preferred or advantageous over other implementations. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary, or the following detailed description.

A magnetoresistive random access memory (MRAM) array includes a metal reset line positioned near each of a plurality of magnetoresistive bits. A write, or program, operation begins when a current is applied to the metal reset line for generating a magnetic field to set the plurality of bits to a reset state. A spin torque transfer (STT) current is then applied to selected ones of the magnetoresistive bits to switch the selected bit to the opposite programmed state. During a read operation, a resistance of each of the plurality of bits is sensed prior to generating a reset magnetic field with the program line. The resistance is again sensed after the magnetic field is generated, changing the state of the bits in the opposite programmed state and thus allowing the initial state of all the bits to be determined. The STT current is then applied to those magnetoresistive bits having a resistance different from prior to the magnetic field being applied, returning them to their initial state. Some embodiments require a STT current be applied in only one direction, and allow for the use of bipolar technology to provide the STT current.

MRAM technology uses magnetic components to achieve non-volatility, high-speed operation, and excellent read/write endurance. FIG. 1 illustrates a conventional memory element array 10 having one or more memory elements 12. An example of one type of magnetoresistive memory element, a magnetic tunnel junction (MTJ) element, comprises a fixed ferromagnetic layer 14 that has a magnetization direction fixed with respect to an external magnetic field and a free ferromagnetic layer 16 that has a magnetization direction that is free to rotate with the external magnetic field. The fixed layer and free layer are separated by an insulating tunnel barrier layer 18. The resistance of memory element 12 relies upon the phenomenon of spin-polarized electron tunneling through the tunnel barrier layer between the free and fixed ferromagnetic layers. The tunneling phenomenon is electron spin dependent, making the electrical response of the MTJ element a function of the relative magnetization orientations and spin polarization of the conduction electrons between the free and fixed ferromagnetic layer.

The memory element array 10 includes conductors 20, also referred to as digit lines 20, extending along rows of memory elements 12, conductors 22, also referred to as bit lines 22, extending along columns of the memory elements 12, and conductor 19, also referred to as a bottom electrode 19, electrically contacting the fixed layer 14. While the bottom electrodes 19 contact the fixed ferromagnetic layer 14, the digit line 20 is spaced from the bottom electrodes 19 by, for example, a dielectric material (not shown). A memory element 12 is located at a cross point of a digit line 20 and a bit line 22. The magnetization direction of the free layer 16 of a memory element 12 is switched by supplying currents to bottom electrode 19 and bit line 22. The currents create magnetic fields that switch the magnetization orientation of the selected memory element from parallel to anti-parallel, or vice versa. To sense the resistance of element 12 during the read operation, a current is passed from a transistor in the substrate (not shown) through a conductive via (not shown) connected to bottom electrode 19.

FIG. 2 illustrates the fields generated by a conventional linear digit line 20 and bit line 22. To simplify the description of MRAM device 10, all directions will be referenced to an x- and y-coordinate system 50 as shown. A bit current I_(B) 30 is defined as being positive if flowing in a positive x-direction and a digit current I_(D) 34 is defined as being positive if flowing in a positive y-direction. A positive bit current I_(B) 30 passing through bit line 22 results in a circumferential bit magnetic field, H_(B) 32, and a positive digit current I_(D) 34 will induce a circumferential digit magnetic field H_(D) 36. The magnetic fields H_(B) 32 and H_(D) 36 combine to switch the magnetic orientation of the memory element 12.

The traditional MRAM switching technique, using magnetic fields generated by current-carrying lines adjacent to the memory element, depicted in FIG. 2, has some practical limitations, particularly when the design calls for scaling the bit cell to smaller dimensions. For example, decreasing the physical size of the MTJ elements results in lower stability against unwanted magnetization reversal due to thermal fluctuations, because the energy barrier to thermal reversal decreases with decreasing free layer volume. The stability of the bit can be enhanced by increasing the magnetic anisotropy which also results in an increased switching field. The magnitude of the magnetic fields H_(D) 36 or H_(B) 32 per milli-Ampere of current I_(D) 34 or I_(B) 30 increases with decreasing line width, which would help reach the higher switching field, but only if the lines are moved proportionally closer to the MTJ, which is not normally possible. In addition, the line resistance increases with decreasing line width, requiring shorter lines which results in lower array efficiency. Furthermore, the bit-to-bit variation in switching field increases with decreasing bit size, requiring even more current to overcome the distributions and achieve reliable switching.

While ST-MRAM is described in detail, some of the programming and reading techniques described herein may also be implemented in field-switched MRAM and phase change memory. In field-switched MRAM, the STT program current would be replaced by current pulses through two write lines that generate magnetic fields to switch the selected bit. In phase change memory, the reset line would be optimized to generate a heat pulse that resets the plurality of bits into a first resistance state, rather than the magnetic field pulse required for the ST-MRAM embodiment. In ST-MRAM devices such as the structure 60 shown in FIG. 3, the bits are written by using isolation transistor 42 to force a current 40 directly through the stack of materials that make up the magnetic tunnel junction 12. Generally speaking, the write current 40 which is spin polarized by passing through one ferromagnetic layer (14 or 16), exerts a spin torque on the subsequent layer. This torque can be used to switch the magnetization of free magnet 16 between two stable states by changing the write current polarity.

Briefly, a current becomes spin-polarized after the electrons pass through the first magnetic layer in a magnet/non-magnet/magnet trilayer structure, where the first magnetic layer is substantially fixed in its magnetic orientation by any one of a number of methods known in the art. The spin-polarized electrons cross the nonmagnetic spacer and then, through conservation of spin angular momentum, exert a spin torque on the second magnetic layer, which switches the magnetic orientation of the second layer to be parallel to the magnetic orientation of the first layer. If a current of the opposite polarity is applied, the electrons instead pass first through the second magnetic layer. After crossing the nonmagnetic spacer, a spin torque is applied to the first magnetic layer. However, since its magnetization is fixed, the first magnetic layer does not switch. Simultaneously, a fraction of the electrons will then reflect off the first magnetic layer and travel back across the nonmagnetic spacer before interacting with the second magnetic layer. In this case, the spin torque acts so as to switch the magnetic orientation of the second layer to be anti-parallel to the magnetic orientation of the first layer. Spin-torque switching occurs only when the current 40 exceeds the critical current I_(c) of the element. The spin-torque switching current used by the circuit is chosen to be somewhat above the average I_(C) of the memory elements so that all elements will switch reliably when the switching current is applied.

Referring to FIG. 4, an exemplary magnetoresistive memory array 400 includes switching devices 404 and 406 formed within and on the substrate 402 in a manner well known in the semiconductor industry. The switching devices 404 and 406 are shown as CMOS transistors, but may alternatively be diodes or bipolar transistors as subsequently described in detail. The switching device 404 includes electrodes 412, 414, channel region 416, and gate 418. The switching device 406 includes electrodes 412, 424, channel region 426, and gate 428. In operation, a voltage VSS is applied to the conductive region 442.

Vias 432, 434, 436 are formed in the dielectric material 430 to couple the electrode 412 to conductive region 442, electrode 414 to conductive region 444, and electrode 424 to conductive region 446. Conductive regions 442, 444, 446 are formed in the same processing steps and isolated from each other by processes known in the semiconductor industry. Conductive line 466 is configured to receive a reset voltage and may be also referred to as a reset line. Vias 452 and 454 are formed in the dielectric material 430 to couple the conductive region 444 to conductive region 462, and conductive region 446 to conductive region 464. In the same process steps in which the conductive regions 462 and 464 are formed, a conductive reset line 466 is formed. The reset line 466 preferably includes magnetically permeable cladding material on three sides of the line, along the length of the line, to focus the magnetic field above the fourth side where the MTJ bits are located.

Scalable magnetoresistive memory elements 472, 474 are formed within the dielectric material 430. When MTJ devices are used, each includes a free layer 482 and a fixed region 485 over a bottom electrode 484 and separated by a tunnel barrier 486. In this illustration, only two magnetoresistive memory elements 472, 474 are shown for simplicity in describing the embodiments of the present invention, but it will be understood that an MRAM array may include many magnetoresistive memory elements. The free layer 482 is coupled to a first conductive line 408, and the fixed regions 485 of scalable magnetoresistive memory elements 472, 474 are coupled to conductive regions 462, 464, respectively by bottom electrode 484 and vias 492, 494.

The fixed magnetic region 485 is well known in the art, and conventionally includes a pinned synthetic antiferromagnet, which includes a ferromagnetic fixed layer, a coupling spacer layer, a pinned ferromagnetic layer and an optional antiferromagnetic pinning layer. The fixed ferromagnetic layer is positioned in contact with the tunnel barrier. The coupling spacer layer is position between the fixed ferromagnetic layer and the pinned ferromagnetic layer. The antiferromagnetic pinning layer is underneath and in contact with the pinned ferromagnetic layer. The ferromagnetic fixed and pinned layers each have a magnetic moment vector that are usually held anti-parallel by the coupling spacer layer and held in a fixed direction by the antiferromagnetic pinning layer. Thus the magnetic moment vector of fixed magnetic region 485 is not free to rotate and is used as a reference. The coupling spacer layer is formed from any suitable nonmagnetic material that creates antiparallel coupling between two ferromagnetic layers adjacent to, and in contact with, its opposite surfaces. The coupling layer is typically one that exhibits the oscillatory coupling phenomenon, for example, at least one of the elements Ru, Os, Re, Cr, Rh, Cu, or their combinations, with a thickness chosen for strong antiparallel coupling. The optional pinning layer, may comprise antiferromagnetic materials such as PtMn, IrMn, FeMn, PdMn, or combinations thereof. However, it will be appreciated by those skilled in the art that fixed magnetic region 484 may have any structure suitable for providing a fixed magnetic portion in contact with the tunnel barrier to provide a fixed magnetic reference direction.

The free layer 482 has a magnetic moment vector that is free to rotate in response to an applied field or a STT current (discussed hereinafter). In the absence of the STT current, the magnetic moment vector is oriented along the anisotropy easy-axis of the free layer.

The free layer 482 easy axis is oriented at an angle in the range of about 30 degrees to about 60 degrees from the direction of the magnetic field generated by the reset line 466. In a preferred embodiment of the invention, the free layer 482 easy axis is oriented at an angle of about 45 degrees from the direction of the magnetic field generated by the reset line 466. It can be easily shown that for a typical free layer having a Stoner-Wohlfarth-like switching astroid, the switching field is reduced by a factor of 2 if the field is applied at 45 degrees from the easy axis of the bit. Therefore, the current needed to reset the bits is also reduced by a factor of 2, making the reset require less power. The magnetic vector of the free layer 482 is free to rotate in the presence of an applied field or a STT current. The magnetic vector of the fixed layer 485 is not free to rotate in the presence of an applied field or an STT current and is used as the reference layer.

The free layer 482, the fixed layer, and the pinned layer may be formed from any suitable ferromagnetic material, such as at least one of the elements Ni, Fe, Co or their alloys and their other useful alloys incorporating other elements, such as B, Ta, V, Si, and C, as well as so-called half-metallic ferromagnets such as NiMnSb, PtMnSb, Fe₃O₄, or CrO₂. The tunnel barrier 486 may be comprised of insulating materials such as AlOx, MgOx, RuOx, HfOx, ZrOx, TiOx, or the nitrides and oxidinitrides of these elements.

During fabrication of the MRAM array architecture 400, each succeeding layer is deposited or otherwise formed in sequence and each MTJ device 400 may be defined by selective deposition, photolithography processing, etching, etc. using any of the techniques known in the semiconductor industry. Typically the layers of the MTJ are formed by thin-film deposition techniques such as physical vapor deposition, including magnetron sputtering and ion beam deposition, or thermal evaporation. During deposition of at least a portion of the MTJ, a magnetic field is provided to set a preferred anisotropy easy-axis into the material (induced intrinsic anisotropy). In addition, the MTJ stack is typically annealed at elevated temperature while exposed to a magnetic field directed along the preferred anisotropy easy-axis to further set the intrinsic anisotropy direction and to set the pinning direction when an antiferromagnetic pinning layer is used. The provided magnetic field creates a preferred anisotropy easy-axis for a magnetic moment vector in the ferromagnetic materials. In addition to intrinsic anisotropy, memory elements patterned into a shape having aspect ratio greater than one will have a shape anisotropy, and the combination of this shape and the intrinsic anisotropy define an easy axis that is preferably parallel to a long axis of the memory element. In some cases it may be advantageous to employ free layer materials that have a strong perpendicular magnetic anisotropy (PMA) such that the easy axis of the free layer is perpendicular to the film plane and the two stable magnetic states are characterized by a magnetization vector directed generally toward or away from the tunnel barrier. Such PMA materials known in the art include certain ordered L10 alloys such as: FePt, FePd, CoPt, FeNiPt; and certain artificial multilayered structures such as: Co/Pt, Co/Pd, CoCr/Pt, Co/Ru, Co/Au, Ni/Cu. If the easy axis of the free layer is perpendicular to the film plane, then the magnetoresistive memory elements 472, 474 will not be centered with respect to the reset lines 466 but rather offset toward one edge of the reset lines 466 so that they experience a significant perpendicular component of the field generated by the reset lines 466.

In the MRAM array architecture 400, the dielectric material 430 may be silicon oxide, silicon nitride (SiN), silicon oxynitride (SiON), a polyimide, or combinations thereof. The conductive lines 408 and vias 432, 434, 436, 452, 454, 492, 494, are preferably copper, but it will be understood that they may be other materials such as tantalum, tantalum nitride, silver, gold, aluminum, platinum, or another suitable conductive material.

Another exemplary MRAM array architecture 500 is shown in FIG. 5, wherein like structures are represented by the same reference number. For the substrate 402, switching devices 404, 406, vias 432, 434, 436, 452, 454, conductive regions 442, 444, 446, 466, and magnetoresistive memory elements 472, 474, the fabrication process and material composition are similar to those of FIG. 4 and are not repeated. Differences between the magnetoresistive memory 500 of FIG. 5 and the magnetoresistive memory 400 of FIG. 4 include the position of the conductive regions 466 (program or reset line) over the magnetoresistive memory elements 472, 474, and the conductive region 442 serving as the bit line. The MRAM array architecture 500 provides a small bitcell having increased field generating capability for a given current through reset line 466 since reset line 466 can be located in closer proximity to magnetoresistive memory elements 472, 474.

Yet another exemplary MRAM array architecture 600 is shown in FIG. 6, wherein like structures are represented by the same reference number. For the substrate 402, switching devices 404, 406, vias 432, 434, 436, 452, 454, conductive regions 442, 444, 446, 466, conductive bit line 408, and magnetoresistive memory elements 472, 474, the fabrication process and material composition are similar to those of FIG. 4 and are not repeated. The conductive region 442 is configured to receive a voltage VSS and the conductive layer 408 serves as the bit line. Differences between the magnetoresistive memory 600 of FIG. 6 and the magnetoresistive memory 400 of FIG. 4 include the position of the reset lines 466 over, and spaced from, the magnetoresistive memory elements 472, 474. The magnetoresistive memory 600 structure provides a small bitcell similar to 500, however, eliminates any negative impact of the voltage drop down reset line 466 on the sensing operation and the application of STT current by electrically isolating reset line 466 from magnetoresistive memory elements 472, 474 (similar to memory 400). If the easy axis of the free layer is perpendicular to the film plane, then the magnetoresistive memory elements 472, 474 will not be centered with respect to the reset lines 466 but rather offset toward one edge of the reset lines 466 so that they experience a significant perpendicular component of the field generated by the reset lines 466.

For the magnetoresistive memories 400, 500, 600 discussed above, a current is applied through the reset line 466 positioned contiguous to each of the plurality of magnetoresistive memory elements 472, 474 to create and apply a magnetic field to the plurality of magnetoresistive memory elements 472, 474 and set each of the magnetoresistive memory elements 472, 474 to a first state. An STT current is then applied to selected ones of the plurality of magnetoresistive memory elements to change the state of the selected magnetoresistive memory elements 472, 474 to a second state. More specifically, referring to the flow chart of FIG. 7, a method is provided for programming an ST-MRAM including setting 702 the plurality of magnetoresistive memory elements 472, 474 to a first state by applying a current through a reset line 466 to apply a magnetic field to each of the plurality of magnetoresistive memory elements 472, 474, and programming 704 selected ones of the plurality of magnetoresistive memory elements 472, 474 to a second state by applying a STT current to the selected magnetoresistive memory elements 472, 474 from circuitry 404, 406 through the bit line 408 (FIGS. 4, 6) and 442 (FIG. 5).

In an alternative embodiment, during reset of the memory elements a current is also passed through the memory elements in addition to the current applied through the reset line 466. The current passing through the memory elements can be large enough to cause magnetic heating of the free layer so that the magnetization of the free layer and the corresponding switching field of the memory element is reduced. With a reduced switching field, the reset field and reset current are reduced, which is desirable for both for lower power and flexibility in circuit design. The magnitude of the heating current can be similar to that required for ST switching, and the polarity can be the same. Although the ST on the free layer opposes the switching torque generated by the magnetic field, the field torque dominates so that the memory elements are reset into the correct state. In a preferred method of this alternative embodiment, the heating current is turned off before the field reset current is reduced to 0.

A method of reading data from the ST-MRAM includes first sensing the resistance of each of the magnetoresistive memory elements 472, 474 followed by a current being applied through the reset line 466 positioned contiguous to each of the plurality of magnetoresistive memory elements 472, 474 to create and apply a magnetic field to the plurality of magnetoresistive memory elements 472, 474 and set each of the magnetoresistive memory elements 472, 474 to a first state. A second sensing is then performed to sense the resistance of the magnetoresistive memory elements 472, 474. The magnetoresistive memory elements 472, 474 that have changed resistance corresponding to a change in magnetic state from the first sensing to the second sensing are identified. The sequence of memory elements that have or have not changed resistance corresponding to a change in magnetic state represents the data that was stored in the magnetoresistive memory elements. An STT current is then applied to the identified magnetoresistive memory elements 472, 474 to change the state of the identified magnetoresistive memory elements 472, 474 to a second state. More specifically, referring the flow chart of FIG. 8, a method is provided for reading a ST-MRAM including sensing 802 a resistance for each of the plurality of magnetoresistive memory elements 472,474, setting 804 the plurality of magnetoresistive memory elements 472, 474 to a first state by applying a current through a reset line 466 to apply a reset field to each of the plurality of magnetoresistive memory elements 472, 474, sensing 806 a change in at least one of the magnetoresistive memory elements that changed magnetic state in response to the reset field, and programming 808 at least one of the magnetoresistive memory elements 472, 474 having the change in resistance to a second state by applying a STT current to the selected magnetoresistive memory element 472, 474 from circuitry 404, 406 through the bit line 408 (FIGS. 4, 6) and 442 (FIG. 5), thus returning the plurality of MTJ bits back to their original states.

A method of operation of the ST-MRAM circuit includes storing the data that has been read from the plurality of magnetoresistive memory elements 472, 474 into associated circuitry, such as a plurality of latches, allowing the user to perform read operations from the circuitry and write operations to the circuitry that may change one or more of the stored states in the stored data, and the writing of those stored states to the plurality of magnetoresistive memory elements 472, 474. More specifically, referring the flow chart of FIG. 9, a method is provided for programming a magnetoresistive memory including sensing 902 a resistance for each of the plurality of magnetic field generator structure 472,474, setting 904 the magnetoresistive memory elements 472, 474 to a first state by applying a current through a reset line 466 to apply a reset field to each of the magnetoresistive memory elements 472, 474, sensing 906 a change in the resistance of any of the plurality of magnetoresistive memory elements 472, 474, identifying 908 and storing data representing the initial states of the magnetoresistive memory elements 472, 474 into associated circuitry, changing 910 one or more of the stored states in the stored data in the associated circuitry, and, in response to the stored data, programming 912 certain magnetoresistive memory elements 472, 474 to a second state as needed to represent the stored data by applying a STT current to the selected magnetoresistive memory elements 472, 474 from circuitry 404, 406 through the bit line 408 (FIGS. 4, 6) and 442 (FIG. 5).

FIG. 10 is a block diagram of a typical sensing circuit 1000 that may be used in the magnetoresistive memories 400, 500, 600 for the method of FIG. 8 to program the magnetoresistive memory elements by sensing the resistance of each of the magnetoresistive bits 472, 474. A sense amplifier 1002 and a write circuit 1004 are coupled to the bit line 408 of FIGS. 4 and 6 and bit line 442 of FIG. 5. The sense amplifier 1002 senses the resistance of the magnetoresistive memory elements 472, 474 and provides data out to the write circuitry 1004, which is used to program the magnetoresistive memory elements 472, 474. For the method of FIG. 9, the sense amplifier includes a pre-amp 1006 and a latch 1008. The resistance sensed by preamp 1006 is stored as data in the latch 1008. A user may change (step 910) the data prior to magnetoresistive memory elements bits being programmed.

The flow charts of FIGS. 7, 8, 9 are examples suitable for use with a magnetoresistive memory. It should be appreciated that the methods may include any number of additional or alternative tasks, the tasks shown and discussed need not be performed in the illustrated order, and additional steps may be incorporated into a more comprehensive procedure or process having additional functionality not described in detail herein. Moreover, one or more of the tasks shown and described could be omitted from one of the methods as long as the intended overall functionality remains intact.

The magnetoresistive memories 400, 500, 600 as shown in FIGS. 4, 5, 6 include a NMOS as the switching devices 404, 406, and as represented in the schematic diagram of FIG. 11. Switching devices 404, 405, are coupled between the bit line BL1 and ground, and switching devices 406, 407 are coupled between bit line BL0. Magnetoresistive memory elements 472, 473 are coupled between bit line BL0 and an electrode of the switching devices 404, 405, respectively. The magnetoresistive memory elements 474, 475 are coupled between bit line BL0 and an electrode of the switching devices 406, 407, respectfully. The gates of switching devices 404, 406 are coupled to the word line WL1, and the gates of switching devices 405, 407 are coupled to the word line WL0 This configuration of switching devices 404, 405, 406, 407 in allows for a uni-directional STT current to program the magnetoresistive memories 400, 500, 600, 700. Also shown are exemplary conditions for voltages applied to BL0, BL1, WL0, and WL1 necessary to apply STT current or a read bias current through magnetoresistive memory element 472 while no current is applied to magnetoresistive memory elements 473, 474, 475. Applying a voltage to the gates of NMOS devices 404 and 406 enables a current path to ground at the bottom electrode of magnetoresistive memory elements 472 and 474, while applying ground to the gates of NMOS devices 405 and 407 maintains an open circuit at the bottom electrodes of magnetoresistive memory elements 473 and 475. In addition, applying a positive voltage, such as VDD, to BL1 causes current to flow through magnetoresistive memory element 472, but not through magnetoresistive memory element 473. Applying ground to BL0 prevents current from flowing through either magnetoresistive memory element 474 or 475.

These switching devices 404, 405, 406, 407 may alternatively comprise diodes 1204, 1205, 1206, 1207 (FIG. 12) or bipolar transistors 1304, 1305, 1306, 1307 (FIG. 13). Similar to FIG. 11, the exemplary voltage conditions shown on BL0, BL1, WL0, and WL1 demonstrate selectively applying current through magnetoresistive memory element 472 while no current is applied to magnetoresistive memory elements 473, 474, and 475. Specifically, in FIG. 12, applying a positive voltage, such as VDD, to BL1 and WL0 while applying ground to BL0 and WL1 causes diode 1204 to be forward biased and conduct current through magnetoresistive memory element 472, while diodes 1205, 1206, and 1207 are reverse biased and no current flows through magnetoresistive memory elements 473, 474, and 475. In FIG. 13, the same voltage conditions as described in FIG. 12 forward bias the base-emitter terminals of BJT 1304 and cause current to flow through magnetoresistive memory element 472 to the collector of BJT 1304 to ground while the base emitter terminals of BJTs 1305, 1306, and 1307 remain reversed biased and no current flows through magnetoresistive memory elements 473, 474, and 475. With these examples, it can be seen how current may be selectively applied to one or more magnetoresistive memory elements in an array containing several rows and several columns of magnetoresistive memory elements.

It is understood that instead of forming the transistors 404, 405, 406, 407, diodes 1204, 1205, 1206, 1207, or bipolar transistors 1304, 1305, 1306, 130 in the substrate as shown in FIGS. 4, 5, 6, they may be formed between metal layers enabling the possibility of multiple vertical layers of memory elements.

It has been shown that the magnetoresistive memories 400, 500, 600 structures and methods of operation therefore enable a faster self-referenced read operation, reduce the power requirement when performing a self-referenced read operation to a plurality of bits, or enable a memory to function with ST-MRAM switching that requires current be applied through the MTJ in only one direction.

While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims. 

1. A method of operating a spin-torque magnetoresistive memory including a bit line coupled to each of a plurality of magnetoresistive memory elements, circuitry coupled to the bit line, and a metal reset line positioned near the plurality of magnetoresistive memory elements, the method comprising: setting the plurality of magnetoresistive memory elements to a first state by applying a reset current through the metal reset line to apply a magnetic field to each of the plurality of magnetoresistive memory elements; and programming selected ones of the plurality of magnetoresistive memory elements to a second state by applying a spin torque transfer current through the selected magnetoresistive memory elements from the circuitry to the bit line.
 2. The method of claim 1 wherein the circuitry comprises a bipolar transistor, and applying the spin torque transfer current step comprises supplying the spin torque transfer current through the bipolar transistor to the bit line.
 3. The method of claim 1 wherein the circuitry comprises a wordline and a diode coupled to the wordline, and the applying a spin torque transfer current step comprises: applying the spin torque transfer current through the diode to the wordline.
 4. The method of claim 1 wherein the magnetoresistive memory elements each comprise a magnetic tunnel junction device with a fixed magnetic region and a free magnetic region separated by a tunnel barrier.
 5. The method of claim 1 further comprising applying a heating current through the bit line while applying the reset current through the metal reset line.
 6. The method of claim 1 wherein an easy axis of the magnetoresistive memory elements are between 30 to 60 degrees to the magnetic field.
 7. The method of claim 1 further comprising: prior to the setting step, sensing a resistance for each of the plurality of magnetoresistive memory elements; and subsequent to the setting step, sensing the resistance for each of the of the magnetoresistive memory elements, wherein the programming step programs only magnetoresistive memory elements having a change in resistance corresponding to a change in magnetic state subsequent to the setting step.
 8. The method of claim 7 wherein the circuitry comprises a bipolar transistor, and applying the spin torque transfer current comprises supplying the spin torque transfer current through the bipolar transistor to the bit line.
 9. The method of claim 7 wherein the circuitry comprises a wordline and a diode coupled to the wordline, and the applying a spin torque transfer current step comprises: applying the spin torque transfer current through the diode to the wordline.
 10. The method of claim 7 further comprising: prior to the programming step, identifying and storing data representing the initial state of each of the magnetoresistive memory elements as determined from the change in resistance caused by the setting step; and enabling the stored data to be read out through additional interface circuitry.
 11. The method of claim 7 further comprising: prior to the programming step, identifying and storing data representing the initial state of each of the magnetoresistive memory elements as determined from the change in resistance subsequent to the setting step; and changing one or more of the stored data, wherein the programming step programs the selected ones of the plurality of magnetoresistive memory elements to a second state in response to the stored data.
 12. The spin-torque magnetoresistive memory of claim 11 wherein the circuitry comprises a bipolar transistor configured to supply the spin torque transfer current.
 13. The spin-torque magnetoresistive memory of claim 11 wherein the circuitry comprises a diode configured to supply the spin torque transfer current.
 14. The spin-torque magnetoresistive memory of claim 11 further comprising sensing circuitry configured to sense the resistance of each of the plurality of magnetoresistive memory elements.
 15. The method of claim 11 wherein the step of changing of one or more of the stored data comprises: using at least a portion of the stored data to detect errors in the stored data; and using additional circuitry to change the stored data to correct the detected errors.
 16. A spin-torque magnetoresistive memory comprising: a plurality of magnetoresistive memory elements; a metal reset line positioned adjacent each of the plurality of magnetoresistive memory elements and configured to set the plurality of magnetoresistive memory elements to a known state by generating a magnetic field when an electrical current of predetermined magnitude and direction flows through the metal reset line; a bit line coupled to the plurality of magnetoresistive memory elements; and circuitry coupled to the bit line and configured to apply a spin torque transfer current through the bit line to selected ones of the plurality of magnetoresistive memory elements.
 17. The spin-torque magnetoresistive memory of claim 16 wherein the circuitry comprises a bipolar transistor configured to supply the spin torque transfer current.
 18. The spin-torque magnetoresistive memory of claim 16 wherein the circuitry comprises a diode configured to supply the spin torque transfer current.
 19. The spin-torque magnetoresistive memory of claim 16 further comprising sensing circuitry configured to sense the resistance change of each of the plurality of magnetoresistive memory elements corresponding to a change of magnetic state upon reset.
 20. The spin-torque magnetoresistive memory of claim 19 further comprising: storage circuitry configured to store the data represented by the magnetic states of the plurality of magnetoresistive memory elements; and interface circuitry configured to enable the stored data to be accessed for read and write operations to the memory.
 21. The spin-torque magnetoresistive memory of claim 20 further comprising: circuitry configured to select at least one of the plurality of magnetoresistive memory elements to be programmed with a spin torque transfer current, causing the selected magnetoresistive memory element to switch from the known state to a programmed state to represent the stored data.
 22. The spin-torque magnetoresistive memory of claim 16 wherein the magnetoresistive memory elements are oriented so their magnetic easy axis is at an angle within the range of 30 to 60 degrees to the magnetic field generated by the reset line.
 23. The spin-torque magnetoresistive memory of claim 16 wherein in each of the magnetoresistive memory elements comprise a magnetic tunnel junction device with a fixed magnetic region and a free magnetic region separated by a tunnel barrier.
 24. A method of operating a resistive memory including a bit line coupled to each of a plurality of resistive memory elements, circuitry coupled to the bit line, and a metal reset line positioned near the plurality of resistive memory elements, the method comprising: resetting the plurality of resistive memory elements to a first state by applying a first current through the metal reset line to apply a temperature profile; and programming selected ones of the plurality of resistive memory elements to a second state by applying a current through the selected resistive memory elements from the circuitry to the bit line. 